1. Field of the Invention
The present invention relates to a back-flow prevention apparatus.
2. Description of the Related Art
Conventionally, an injection molding machine includes an injection having a heating cylinder in which a screw is disposed rotatably and in an advancingly-retreatively movable manner. Drive means rotates and advances or retreats the screw. In a metering step, the screw is retreated while being rotated, so that resin is dropped from a hopper, melted and accumulated in a space located ahead of the screw. In an injection step, the screw is advanced so as to inject the resin melt into a mold from an injection nozzle.
FIG. 1 shows a sectional view of a main portion of a conventional injection unit.
In FIG. 1, reference numeral 11 denotes a heating cylinder. The heating cylinder 11 has an injection nozzle 13 at its front end (left-hand end in FIG. 1). In the heating cylinder 11, a screw 12 is disposed rotatably and in an advancingly-retreatively movable manner. Unillustrated drive means rotates and advances or retreats the screw 12. Notably, an injection cylinder, a motor, or a like device normally serves as the drive means.
The screw 12 extends rearward (to the right in FIG. 1) within the heating cylinder 11. The screw 12 is connected at its rear end to the drive means and has a screw head 14 at its front end. A spiral flight 15 is formed on the surface of a metering portion 18 of the screw 12 to thereby form a groove 16 along the flight 15.
An unillustrated hopper is disposed at a predetermined position located at a rear portion of the heating cylinder 11. Resin pellets are charged into the hopper.
In the thus-configured injection unit, in a metering step, the drive means causes the screw 12 to rotate and retreat. Resin pellets contained in the hopper drop into the heating cylinder 11 and are advanced (to the left in FIG. 1) through the groove 16.
An unillustrated heater is disposed around the outer circumference of the heating cylinder 11. The heater heats the heating cylinder 11 so as to melt resin pellets contained in the groove 16. Accordingly, when the screw 12 is retreated by a predetermined amount while being rotated, a predetermined amount of resin melt to be injected is stored in a space located ahead of the screw 12.
Upon completion of the metering step, suck-back is performed, i.e., the screw 12 is slightly retreated without being rotated, to thereby prevent the resin from oozing from the front end of the injection nozzle 13.
Next, in an injection step, the drive means is activated so as to advance the screw 12. The resin stored in the space located ahead of the screw 12 is injected into an unillustrated mold cavity from the injection nozzle 13, thereby filling the cavity with the resin.
In order to prevent backflow of the resin stored in the space located ahead of the screw 12 in the injection step, a back-flow prevention device is provided.
Specifically, the screw head 14 has a conical head body portion 21 formed at its front section and a small-diameter portion 19 formed at its rear section. The small-diameter portion 19 has an unillustrated external thread which extends rearward. The external thread is engaged with an unillustrated internal thread formed in a front end portion of the screw 12 so as to fixedly engage the screw head 14 with the screw 12. An annular back-flow prevention ring 20 is disposed around the circumference of the small-diameter portion 19, thereby defining a resin passageway 24 between the small-diameter portion 19 and the back-flow prevention ring 20. A seal ring 22 is disposed at the front end of the metering portion 18 such that the seal ring 22 can contact or separate from the rear end of the back-flow prevention ring 20.
Accordingly, in the injection step, when the screw 12 is advanced, the resin stored in the space located ahead of the screw 12 is urged to move rearward. However, resin pressure causes the back-flow prevention ring 20 to move rearward with respect to the screw 12. Thus, the rear end of the back-flow prevention ring 20 abuts the seal ring 22 to thereby effect sealing. As a result, the resin stored in the space located ahead of the screw 12 is prevented from flowing rearward.
In contrast, in the metering step, when the screw 12 is retreated while being rotated, resin pressure causes the back-flow prevention ring 20 to move forward with respect to the screw 12. Thus, the front end of the back-flow prevention ring 20 abuts the rear end of the head body portion 21. Since axially extending cuts 25 are formed in the head body portion 21 in a plurality of circumferential positions, resin flow is not hindered.
The conventional back-flow prevention apparatus effects sealing in the injection step through advancing the screw 12 so as to bring the rear end of the back-flow prevention ring 20 into contact with the seal ring 22. Accordingly, the resin flows rearward in an amount corresponding to an advancement of the screw 12 effected before sealing is completed.
Further, in order for a sufficient amount of resin to flow into the resin passageway 24 from the space defined between the back-flow prevention ring 20 and the seal ring 22, the small-diameter portion 19 is made sufficiently long as compared to the back-flow prevention ring 20. Thus, the amount of movement of the back-flow prevention ring 20 for effecting sealing increases accordingly, resulting in an increase in the amount of back-flow resin.
Also, timing for completion of sealing varies depending on the state of kneading and dispersion of resin, resin viscosity, resin temperature, etc., and molding conditions for setting the acceleration to a predetermined injection speed at the time of starting injection, or even under the same molding conditions.
Accordingly, the quantity of back-flow resin varies; consequently, an injection peak pressure varies. As a result, molded products suffer short shot, burrs, or like defects.
Further, when the metering step is started, cancellation or breakage of sealing requires a relatively long time, and excess heat is generated due to plasticization of resin.
Thus, there is provided a back-flow prevention apparatus in which the screw 12 is once rotated in a reverse direction upon completion of the metering step so as to bring the rear end of the back-flow prevention ring 20 into contact with the seal ring 22 to thereby effect sealing.
However, this type of back-flow prevention apparatus requires a mechanism for bringing the rear end of the back-flow prevention ring 20 into contact with the seal ring 22. This not only complicates the structure of the screw head 14 but also involves an additional reverse rotation of the screw 12, making control of the drive means complicated.
To cope with the above-mentioned problems, there is provided a back-flow prevention apparatus using a ball check type check valve.
FIG. 2 shows a back-flow prevention apparatus using a ball check type check valve.
In FIG. 2, reference numeral 11 denotes a heating cylinder, and numeral 32 denotes a screw. The screw 32 includes a metering portion 33 and a screw head 34. The screw head 34 is screw-engaged to the metering portion 33 and its outer diameter is equal to that of the metering portion 33. The screw 32 extends rearward (to the right in FIG. 2) within the heating cylinder 11. A spiral flight 35 is formed on the surface of the metering portion 33 to thereby form a groove 36 along the flight 35.
The screw head 34 has a resin passageway 41 and a valve chamber 43 formed therein. The resin passageway 41 opens at its front end (the left end in FIG. 2), extends rearward, and communicates at its rear end with the valve chamber 43. The metering portion 33 has a resin passageway 46 and resin passageways 47 formed therein. The resin passageway 46 opens at its front end and extends rearward. The resin passageways 47 establish communication between the resin passageway 46 and the groove 36. A valve seat 45 is formed at the front end of the resin passageway 46. A ball 49 serving as a check valve is pressed against the valve seat 45 by an urging force of a coil spring 50.
In the thus-configured back-flow prevention apparatus, in a metering step, unillustrated drive means causes the screw 32 to rotate and retreat. Resin pressure causes the ball 49 to advance (move to the left in FIG. 2) against the urging force of the coil spring 50. Accordingly, resin advanced through the groove 36 passes sequentially through the resin passageways 47 and 46, the valve chamber 43, and the resin passageway 41 and is then stored in a space located ahead of the screw 32.
Subsequently, in an injection step, when the screw 32 is advanced, the resin stored in the space located ahead of the screw 32 is urged to flow rearward. However, the urging force of the coil spring 50 causes the ball 49 to be pressed against the valve seat 45, thereby effecting sealing. As a result, the resin stored in the space located ahead of the screw 32 is prevented from flowing rearward.
Accordingly, the time required for charging becomes shorter, so that resin temperature is prevented from dropping and resin viscosity is prevented from increasing during the charging. Also, since the screw head 34 does not include a small-diameter portion, potential breakage of the screw head 34 can be prevented in an initial state of molding in which resin begins to plasticize.
Also, upon start of the metering step, breakage of sealing is completed promptly, so that excess heat is not generated due to plasticization of resin.
However, since the coil spring 50 is used for effecting sealing, not only is a relatively large stress borne by the coil spring 50, but also the coil spring 50 comes into contact with high-temperature high-pressure resin. Accordingly, the coil spring 50 deteriorates in a relatively short period of time, resulting in reduced durability of the back-flow prevention apparatus. Further, since the resin passageways 41, 46, and 47 are narrow and in a complicated form, shearing heat is generated while resin flows through the resin passageways 41, 46, and 47.